Edit existing code

Make surgical changes to your project

⏱ Est. ~7 min

01 · Read

Creating new files is great, but most real-world dev is editing existing code. Adding features to apps, fixing bugs, updating designs, refactoring functions.

This is where Claude Code really shines. Because it can read your whole project, it understands context. Ask it to add a footer and it knows what HTML structure exists. Ask it to change styling and it knows which CSS file to edit.

Claude doesn't start from scratch — it makes surgical edits to exactly what needs changing, and leaves the rest alone.

💡 Picture thisImagine asking a contractor to add a room to your house. A bad contractor might knock down random walls. A good contractor studies the blueprints, understands the structure, and adds the room exactly where it belongs. Claude Code is the good contractor.

02 · Prompt template

Ask Claude to add a footer to the HTML you made in the last lesson. Be specific about what goes in it.

Add a footer to index.html with a copyright 2024 notice and a link to GitHub

03 · Prompt template

Now ask Claude to change the CSS. This is a precise edit — Claude should only update the relevant style rules.

Make the nav bar sticky so it stays at the top when you scroll

04 · Real-machine exercise

Now use the tool you learned in Level 4 — git diff. It shows exactly what Claude changed. Green is added, red is removed.

git diff

05 · Read

Here's the key habit: always review changes before committing. Claude is powerful but not perfect. You should always: - Read the diff to understand what changed - Test the change in the browser or by running the code - Commit only when you're happy

git diff is your best friend. It tells you, line by line, what was added, removed, and changed.

Key points

Always review changes before committing
git diff tells you what Claude changed
Green (+) = added, red (-) = removed
Claude makes surgical edits — only changes what's needed

06 · Quiz

After Claude edits your code, what command tells you exactly what changed?

git status
git log
git diff
git show

07 · Fill in the blank

Before committing, to see the exact lines Claude changed, run git ___.

Other lessons in this chapter

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※ This is an independent Traditional Chinese teaching project — not an official Anthropic product. Claude™ is a trademark of Anthropic, PBC.